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Background and aims While the coupled effects of root exudates and microbial feedbacks on soil processes are well-recognized, we still lack an understanding of differences in root exudate fluxes and the associated ecological consequences among tree growth forms. Methods Two deciduous tree species (i.e., Cercidiphyllum japonicum and Larix kaempferi ) and two evergreen tree species (i.e., Pinus armandi and Pinus tabulaeformis ) were selected to perform an in-situ collection of root exudates during the growing season in 2016. The net N mineralization rates and associated microbial enzyme activities were measured in rhizosphere and bulk soils to evaluate rhizosphere effects. Moreover, we compiled the dataset related to root exudation and their associated biological traits and the soil chemical properties for 21 tree species from temperate forests. Results The root exudation rates and the annual root exudate carbon (C) fluxes of two deciduous tree species were significantly higher than those of the two evergreen tree species. Correspondingly, the rhizosphere effects of deciduous tree species on the microbial biomass, enzyme activity and net N mineralization rate were approximately 1.9, 1.6 and 2.4 times greater than those of the evergreen tree species, respectively. Rhizosphere effects were positively correlated with the root exudation rate. The compiled dataset also suggest that deciduous tree species tend to have higher exudation rates than evergreen tree species in temperate forests. Conclusions Collectively, these results suggest that the two tree growth forms exhibit different patterns in root exudate inputs and associated rhizosphere microbial processes. Generally, deciduous tree species tend to exude more C into the soil and consequently induce greater microbial feedback on soil N transformations during the growing season in temperate regions, implying that deciduous tree species induced a greater effect on the C and nutrient cycling in rhizosphere soil than evergreen tree species.

Plants produce a wide array of secretions both above and below ground. Known as mucilages or exudates, they are secreted by seeds, roots, leaves and stems and fulfil a variety of functions including adhesion, protection, nutrient acquisition and infection. Mucilages are generally polysaccharide‐rich and often occur in the form of viscoelastic gels and in many cases have adhesive properties. In some cases, progress is being made in understanding the structure–function relationships of mucilages such as for the secretions that allow growing ivy to attach to substrates and the biosynthesis and secretion of the mucilage compounds of the Arabidopsis seed coat. Work is just beginning towards understanding root mucilage and the proposed adhesive polymers involved in the formation of rhizosheaths at root surfaces and for the secretions involved in host plant infection by parasitic plants. In this article, we summarise knowledge on plant exudates and mucilages within the concept of their functions in microenvironmental design, focusing in particular on their bioadhesive functions and the molecules responsible for them. We draw attention to areas of future knowledge need, including the microstructure of mucilages and their compositional and regulatory dynamics.

Microbial communities associated with roots confer specific functions to their hosts, thereby modulating plant growth, health, and productivity. Yet, seminal questions remain largely unaddressed including whether and how the rhizosphere microbiome modulates root metabolism and exudation and, consequently, how plants fine tune this complex belowground web of interactions. Here we show that, through a process termed systemically induced root exudation of metabolites (SIREM), different microbial communities induce specific systemic changes in tomato root exudation. For instance, systemic exudation of acylsugars secondary metabolites is triggered by local colonization of bacteria affiliated with the genus Bacillus. Moreover, both leaf and systemic root metabolomes and transcriptomes change according to the rhizosphere microbial community structure. Analysis of the systemic root metabolome points to glycosylated azelaic acid as a potential microbiome-induced signaling molecule that is subsequently exuded as free azelaic acid. Our results demonstrate that rhizosphere microbiome assembly drives the SIREM process at the molecular and chemical levels. It highlights a thus-far unexplored long-distance signaling phenomenon that may regulate soil conditioning.

Key message This article describes the composition of root exudates, how these metabolites are released to the rhizosphere and their importance in the recruitment of benefcial microbiota that alleviate plant stress. Abstract Metabolites secreted to the rhizosphere by roots are involved in several processes. By modulating the composition of the root exudates, plants can modify soil properties to adapt and ensure their survival under adverse conditions. They use several strategies such as (1) changing soil pH to solubilize nutrients into assimilable forms, (2) chelating toxic compounds, (3) attracting benefcial microbiota, or (4) releasing toxic substances for pathogens, etc. In this work, the composition of root exudates as well as the diferent mechanisms of root exudation have been reviewed. Existing methodologies to collect root exudates, indicating their advantages and disadvantages, are also described. Factors afecting root exudation have been exposed, including physical, chemical, and biological agents which can produce qualitative and quantitative changes in exudate composition. Finally, since root exudates play an important role in the recruitment of mycorrhizal fungi and plant growth-promoting rhizobacteria (PGPR), the mechanisms of interaction between plants and the benefcial microbiota have been highlighted.

Root exudates play an important role in ecosystem response to climate change, but the functional consequences of drought-induced changes in the quality of root exudates are unknown. Here, we addressed this knowledge gap in a unique experimental approach. We subjected two common grassland species that differ widely in their growth strategies and root systems, the grass Holcus lanatus and the forb Rumex acetosa, to 2 wk of drought. We collected root exudates and soils at the end of the drought and after 2 wk of recovery and readded all root exudates to all soils in a fully reciprocal set-up to measure root-exudate-induced respiration. We found that soil treatment was unimportant for determining root-exudate-induced respiration. By contrast, root exudates collected from plants that had experienced drought clearly triggered more soil respiration than exudates from undroughted plants. Importantly, this increased respiration compensated for the lower rates of root exudation in droughted plants. Our findings reveal a novel mechanism through which drought can continue to affect ecosystem carbon cycling, and a potential plant strategy to facilitate regrowth through stimulating microbial activity. These findings have important implications for understanding plant and ecosystem response to drought.

Root exudates are a pathway for plant–microbial communication and play a key role in ecosystem response to environmental change. Here, we collate recent evidence that shows that plants of different growth strategies differ in their root exudation, that root exudates can select for beneficial soil microbial communities, and that drought affects the quantity and quality of root exudation. We use this evidence to argue for a central involvement of root exudates in plant and microbial response to drought and propose a framework for understanding how root exudates influence ecosystem form and function during and after drought. Specifically, we propose that fast-growing plants modify their root exudates to recruit beneficial microbes that facilitate their regrowth after drought, with cascading impacts on their abundance and ecosystem functioning. We identify outstanding questions and methodological challenges that need to be addressed to advance and solidify our comprehension of the importance of root exudates in ecosystem response to drought.

Plants host a mesmerizing diversity of microbes inside and around their roots, known as the microbiome. The microbiome is composed mostly of fungi, bacteria, oomycetes, and archaea that can be either pathogenic or beneficial for plant health and fitness. To grow healthy, plants need to surveil soil niches around the roots for the detection of pathogenic microbes, and in parallel maximize the services of beneficial microbes in nutrients uptake and growth promotion. Plants employ a palette of mechanisms to modulate their microbiome including structural modifications, the exudation of secondary metabolites and the coordinated action of different defence responses. Here, we review the current understanding on the composition and activity of the root microbiome and how different plant molecules can shape the structure of the root-associated microbial communities. Examples are given on interactions that occur in the rhizosphere between plants and soilborne fungi. We also present some well-established examples of microbiome harnessing to highlight how plants can maximize their fitness by selecting their microbiome. Understanding how plants manipulate their microbiome can aid in the design of next-generation microbial inoculants for targeted disease suppression and enhanced plant growth.

• Root exudation stimulates microbial decomposition and enhances nutrient availability to plants. It remains difficult to measure and predict this carbon flux in natural conditions, especially for mature woody plants. Based on a known conceptual framework of root functional traits coordination, we proposed that root functional traits may predict root exudation. • We measured root exudation and other seven root morphological/chemical/physiological traits for 18 coexisting woody species in a deciduous-evergreen mixed forest in subtropical China. • Root exudation, respiration, diameter and nitrogen (N) concentration all exhibited significant phylogenetic signals. We found that root exudation positively correlated with competitive traits (root respiration, N concentration) and negatively with a conservative trait (root tissue density). Furthermore, these relationships were independent of phylogenetic signals. A principal component analysis showed that root exudation and morphological traits loaded on two perpendicular axes. • Root exudation is a competitive trait in a multidimensional fine-root functional coordination. The metabolic dimension on which root exudation loaded was relatively independent of the morphological dimension, indicating that increasing nutrient availability by root exudation might be a complementary strategy for plant nutrient acquisition. The positive relationship between root exudation and root respiration and N concentration is a promising approach for the future prediction of root exudation.

Background To improve plant phosphorus (P)-acquisition efficiency to secure sustainable food production, an important step is to increase the concentration of plant-available P in the rhizosphere. Root exudation of organic anions is a key strategy in mobilizing less-available soil P. Scope This review covers how organic anions (carboxylates) mobilize soil P and research methodologies applied. It then discusses the root-release of organic anions induced by low P availability and their contribution to soil P mobilization and plant P acquisition, and highlights the impact, challenges and perspectives in this research area. Conclusions The release of organic anions is increased considerably in some plant species, but very little in others under low P availability. Rhizosphere organic anions play important roles in increasing plant-available P, but the contribution is greatly affected by many factors. In future research, improved and ecologically meaningful root exudation sampling methods, the use of mature leaf manganese (Mn) concentration or total 14 C exudation as a proxy for rhizosphere carboxylates, case-by-case field experiments, molecular mechanisms underpinning organic anion biosynthesis and efflux under low P availability warrant further attention. Finally, carbon costs and multiple root trait combinations (e.g., root hairs plus root exudation) should be considered in crop breeding programs to generate more P-efficient cultivars.

Abstract Plant–microbiome symbiotic interactions play a crucial role in regulating plant health and productivity. To establish symbiotic relationships, the plant secretes a variety of substances to facilitate microbial community recruitment and assembly. In recent years, important progress has been made in studying how plant exudates attract beneficial microorganisms and regulate plant health. However, the mechanisms of plant exudates-mediated microbial community recruitment and assembly and their effects on plant health are no comprehensive review. Here, we summarize the interaction mechanisms among plant exudates, microbial community recruitment and assembly, and plant health. First, we systematically evaluate the type and distribution of plant exudates, as well as their role in microbiome recruitment and assembly. Second, we summarize the mechanisms of plant exudates in terms of microbiome recruitment, diversity regulation and chemotaxis. Finally, we list some typical examples for elucidating the importance of plant exudates in promoting plant health and development. This review contributes to utilizing plant exudate or beneficial microbiome resources to manage plant health and productivity. This review explores how plant exudates facilitate the recruitment and assembly of beneficial microbial communities, enhancing plant health and productivity through mechanisms like nutrient provision, chemical signaling, and pathogen suppression, while emphasizing their potential in sustainable plant health management strategies.